Well, maybe I should've started with a compression test... I took the intake manifold off, there's some nasty looking goo under cylinder 4 (as in "P0304 engine code"), and some under cylinder 3 as well... Am I looking at replacing the head gasket, or could this be stuff leaking from the injectors or the intake manifold gasket? (The gasket was pretty oily when I took it off, and some bolts that attach the manifold to the head felt loose when I was removing them).

A few comments on replacing the manifold: the bottom attachment was a 13mm nut, not a 12mm bolt as some reports say, took me while to realize that. I couldn't imagine loosening it reaching from the top (maybe possible but too much stuff like wires and vacuum lines around it, not to mention the throttle cable with its brittle attachment and the brake cylinder). The biggest obstacle to get to it from below is the wiring from the transmission that runs just under it. I removed two 10mm bolts attaching the harness and loosened a third one (too hard to remove as the engine mount is in the way). Also removed one 12mm bolt attaching the harness to the engine block and unhooked the connectors from the bracket on the transmission (did not open them up, just moved them). That created just enough space to get a standard 3/8" 13mm socket with a universal joint and a long extension (I used a 10" plus a 6") to the nut and break it loose. The bolt is on the intake manifold, and you only need to loosen the nut enough to move it out of its slot.

Before undoing any bolts on the manifold I would recommend removing the plastic vacuum tree just about the vaccuum booster hose (attached with a single 10mm bolt -- remove the bolt and pull the thing out); I tried to do it after and I felt that maybe it wasn't the best way around it. Old vacuum hoses get pretty brittle and crack easily, better to leave them in place. The brake booster hose can of course be disconnected at the booster, then you can wiggle it out to get out with the manifold.

After getting this step done I'd say that pulling the intake manifold on a 6-cylinder whiteblock is not that much more work than on my B230FD. (My 1994 940 has an EGR valve, at least the 6 cylinder is free of that hassle.)